The present invention generally relates to fuel-fired heating apparatus and, in a preferred embodiment thereof, more particularly relates to a self-powered low NOx burner/fuel-air delivery system representatively incorporated in a fuel-fired natural draft water heater.
Residential gas-fired water heaters are required to produce less emissions of NOx compounds for certain Air Quality Management Districts (AQMD""S) of California and Texas. Present gas-fired water heaters are generally non-powered (i.e., natural draft) appliances and the marketplace requires replacement water heaters to be xe2x80x9cdrop-inxe2x80x9d appliances which precludes adding electrical service to installations. Contemporary non-powered low NOx emission burners are limited in such a way that their air/fuel ratios remain fixed in operation, with size constraints generally limiting the amount of primary aeration deliverable to the burner. Their operation thus tends to be less flexible within semi-sealed systems from the standpoint of reducing their NOx emissions by increasing primary aeration thereto. Powered burner systems have been demonstrated in many examples as producing less NOx emissions. However, providing additional electrical service to a gas-fired water heater imposes additional burdens on the consumer and becomes a barrier to rapid replacement Of the water heater.
From the foregoing it can be seen that it would be desirable to provide a fuel-fired water heater having a self-powered low NOx combustion system that does not have the operating limitations and reliability issues of non-powered burners but provides the functionality of a powered burner without the use of external power. Additionally, it would be desirable to provide such a self-powered combustion system which, in the event in the failure of its self-powering portion, would continue to operate in a conventional non-powered mode until corrective action could be taken.
In carrying out principles of the present invention, in accordance with a preferred embodiment thereof, a specially designed fuel-fired low NOx heating apparatus is provided which is representatively a fuel-fired, natural draft water heater but could alternatively be another fuel-fired heating apparatus such as, for example, a boiler or a furnace.
The water heater has a water storage tank, a combustion chamber, a fuel burner disposed within the combustion chamber, and a flue communicated with the combustion chamber and extending through the tank. According to a key feature of the invention, the water heater is provided with a specially designed self-powered fuel-air delivery system for delivering fuel and combustion air to the burner for combustion thereby to form combustion gases which are received and discharged by the flue which transfers combustion gas heat to water stored in the tank.
The fuel-air delivery system includes a fuel supply structure operative to discharge a quantity of fuel received from a source thereof, a first flow path for receiving the discharged fuel and a first quantity of combustion air and flowing the received fuel and air to the burner, a thermoelectric generator positioned to be heated by the burner during firing thereof, a second flow path through which a second quantity of combustion air may be delivered to the burner, and a fan structure preferably disposed externally of the combustion chamber and operable by the thermoelectric generator to deliver at least one of the first and second quantities of combustion air to the burner. According to a feature of the invention, the fuel-air delivery system is configured in a manner such that its associated fuel-fired heating apparatus remains operable even if either or both of the thermoelectric generator and the fan structure fail to function.
In a first representative embodiment of the water heater, in which the NOx emissions of the water heater are reduced by increasing the primary aeration of the burner, the first flow path is defined by a fuel-air mixing duct extending into and through the combustion chamber to an inlet portion of the burner, the second flow path is defined by an auxiliary combustion air duct extending into the combustion chamber and being connected to the fuel-air mixing duct, and the thermoelectrically driven fan structure is coupled to the auxiliary combustion air duct and is operative to flow the second, auxiliary quantity of combustion air therethrough into the fuel-air mixing duct.
In a second representative embodiment of the water heater, in which the NOx emissions of the water heater are also reduced by increasing the primary aeration of the burner, the first flow path is defined by a fuel-air mixing duct extending into and through the combustion chamber to an inlet portion of said burner, the auxiliary combustion air duct is eliminated, the thermoelectrically driven fan structure is connected in the fuel-air mixing duct, and all of the second flow path extends through the interior of said fuel-air mixing duct.
In a third representative embodiment of the water heater, in which the NOx emissions of the water heater are lowered by both (1) increasing the primary aeration of the burner and (2) providing for flue gas recirculation to the burner, the first flow path is defined by a fuel-air mixing duct extending into and through the combustion chamber to an inlet portion of the burner, the fan structure is coupled to said fuel-air mixing duct, the second flow path extends through said first flow path, and the water heater further comprises an auxiliary flue gas recirculating duct extending through the combustion chamber, coupled to the fuel-air mixing duct, and operative to flow into the fuel-air mixing duct a quantity of combustion gases created by the burner during firing thereof. The inlet of the auxiliary flue gas recirculating duct may be disposed within the combustion chamber or positioned within the flue.
In a fourth representative embodiment of the water heater, in which the NOx emissions of the water heater are lowered using a staged combustion technique, the first flow path is defined by a fuel-air mixing duct extending into and through the combustion chamber to an inlet portion of the burner, the second flow path is defined by an auxiliary combustion air supply duct which is not connected to the fuel-air mixing duct but extends into the combustion chamber to adjacent a secondary combustion zone near the burner, and the thermoelectrically driven fan is connected in the auxiliary combustion air supply duct to flow the second quantity of combustion air therethrough, during firing of the burner, into the secondary combustion zone.